This is a Phase I/II, multicenter randomized, open label trial of a standard therapeutic regimen in current use versus experimental therapies administered over 48 weeks. At least 200 HIV-infected children ages four months to 17 years of age will be enrolled. The children will be stratified by CD4+% and by age to receive one of four treatment regimens. These treatment regimens are A: d4T/nevirapine/ritonavir, B: d4T/3TC/nelfinavir, C: d4T/nevirapine/nelfinavir or D: d4T/3TC/ nevirapine/nelfinavir for 48 weeks. The first 35 subjects per arm will be evaluated with special immunologic studies LPA and extended panel immunophenotyping. Baseline and study safety evaluations will include the monitoring of adverse experiences clinical laboratory tests, physical examinations and vital signs. An assessment of virologic efficacy will occur at the 12 week interim analysis time period. Real-time HIV-1 RNA measurements will occur at weeks 12, 24 and 36. This study will also include two substudies: ACTG 725 A Pharmacokinetic Study of BID Nelfinavir's Dosing Given to Children <30 kg and ACTG 727 An Immunology Substudy of DtaP Vaccine Given to Children 2-9 Years of Age with Negative Tetanus Antibody Titers in ACTG 377.